1. Technical Field
The present invention relates to a mobile communication network system, in particular, to a transmission control method for a vehicular ad hoc network system and a communication apparatus using the same.
2. Description of Related Art
The vehicular ad hoc network (VANET) refers a wireless ad hoc network concept that is derived from the mobile ad hoc network (MANET). The VANET is designed to provide data communication (e.g., video information, driving information, or the like) among nearby vehicles or between vehicles and roadside units in a wirelessly manner through on-board communication units installed on vehicles. The VANET can improve driving efficiency and the driving safety of vehicles, as well as prevent traffic accident in advance. The VANET has been widely used for constructing the traffic management system, the traveler information system, the vehicle control and safety system, the emergency management system, or the like.
The VANET uses moving vehicles or fixed roadside units as transmission nodes and wireless communication technology to form a mobile network for performing point-to-point transmission. Currently, the VANET adopts the transmission control protocol (TCP) and the split-TCP for vehicle-to-vehicle (V2V) communication or the vehicle-to-infrastructure (V2I) communication.
Please refer to FIG. 1A and FIG. 1B. FIG. 1A shows a diagram illustrating an operation of a VANET using a conventional TCP protocol. FIG. 1B shows a diagram illustrating an operation of the VANET using a split-TCP protocol.
As shown in FIG. 1A, the conventional TCP protocol can be used to provide a reliable transmission between a source node (i.e. the vehicle C1) and a destination node (i.e. the vehicle C10) in the VANET. The vehicle C1 utilizes a multi-hop transmission technique and forwards a packet to the vehicle C10 through a plurality of the relay nodes (i.e. the vehicle C2 to the vehicle C9) in sequence. Due to the movement of vehicle, the VANET system generally has fast changing network topology (e.g., the positions of nodes). However, the conventional TCP protocol is a communication protocol suitable for establishing communication with stationary nodes. In other words, the conventional TCP protocol is unable to adapt to the rapid network change of the VANET system. For example, when any vehicle leaves the VANET, the network connection between the source node (i.e. the vehicle C1) and the destination node (i.e. the vehicle C10) is instantly interrupted and the source node (i.e. the vehicle C1) has to search and establish a new network connection as well as retransmits the packet to the destination node (i.e. the vehicle C10), thereby introduces the end-to-end delays. Moreover, for the VANET that adopts the conventional TCP protocol, the channel capturing time associated with one channel could be greatly increased due to long transmission channel and high packet loss rate, thereby generating the channel capturing effect and decreasing the transmission efficiency of using the conventional TCP protocol in the VANET.
The split-TCP technique is brought to resolve the long transmission channel issue associated with the conventional TCP protocol. The split-TCP protocol places proxy nodes (e.g., the vehicles C4 and C7) at designated position and divides the network connection between the source node (i.e. the vehicle C1) and the destination node (i.e. the vehicle C10) into sub-connections SUB_1˜SUB_3. Particularly, each of the sub-connections contains a fixed number of transmission nodes so that the number of hops (i.e. hop paths H1-H3) in each of the transmission paths is fixed. Taking a sub-connection SUB_1 as an example, the packet transmitted from the source node (i.e. the vehicle C1) to the proxy node (i.e. the vehicle C4) is routed through the hop path H1, the vehicle C2, the hop path H2, the vehicle C3, and the hop path H3.
Therefore, when any of the proxy nodes (e.g., the vehicle C5) leaves the VANET, the vehicle C4 (i.e. the proxy node) reestablishes a reliable transmission between the vehicle C4 and the destination node (i.e. the vehicle C10) without needing to reestablish the reliable transmission from the source node (i.e. the vehicle C1) to the destination node (i.e. the vehicle C10). Additionally, whenever a packet is lost during the packet transmission, the packet can be directly retransmitted from the preceding proxy node, thereby improve the packet transmission efficiency of the VANET.
However, since the distance between the source node (i.e. the vehicle C1) and the destination node (i.e. the vehicle C10) is fixed. Each proxy node (e.g., the vehicle C4, C7) further needs to temporarily store the packet and establishes a sub-connection with the subsequent proxy node or the destination node upon receiving the data packet. Thus, the more proxy nodes employed in a network connection (e.g., being divided into more sub-connections), the slower the transmission speed. In other words, the split-TCP protocol employs the proxy node to fixedly divide the connection into designated number of sub-connections. Hence the split-TCP protocol is still unable to adaptively to the rapid network change of the VANET.